Ferroelectric liquid crystal cells

ABSTRACT

A ferroelectric liquid crystal cell is provided with one alignment layer (14) constituted by rubbed nylon which promotes planar alignment in a specific azimuthal direction and the other alignment layer (15) constituted by a polymer with flexible side chains which promotes planar alignment without any preferred azimuthal direction.

BACKGROUND OF THE INVENTION

This invention relates to ferroelectric liquid crystal cells, and inparticular to a method of obtaining a preferred alignment of the liquidcrystal molecules within such cells.

The conventional molecular alignment required for operation of aferroelectric liquid crystal cell is one in which the smectic layers areformed in planes orthogonal to the major surfaces of the cell, sucharrangements sometimes being termed `bookshelf geometry`.

In suitable circumstances bookshelf geometry can be obtained for amaterial exhibiting the phase sequence

    I-N*-S.sub.A -S.sub.C *

by slow cooling of a cell that has rubbed major surfaces that promoteplanar alignment of the molecules in the nematic phase. (At least someof the individual molecules of the liquid crystal medium have to have achiral centre for the medium to be ferroelectric in the tilted phase.The presence of a chiral component in a nematic phase normally induces aregular helical structure and the phase is termed cholesteric. A helicalstructure would interfere with the requisite alignment in the smecticphases and so is effectively eliminated by the use of suitablecompensating chiral constituents of opposite handedness to produce aphase, commonly designated N*, in which any residual cholesteric pitchis large compared with cell thickness. As the material cools into the N*phase the molecules assume planar alignment, with the molecular directorlying in the rubbing direction, and this alignment is preserved as thematerial enters the S_(A) phase. The smectic layers that are thenproduced lie in planes extending orthogonally with respect to the planesof the major surfaces of the cell, and on further cooling into theS_(C) * phase it is intended that the alignment of these smectic layersshall be substantially preserved while the directors of the moleculesare rotated through a small angle to produce a tilted smectic phase.

In practice uniform alignment of the smectic A phase layers can beachieved quite easily when the major surfaces of the liquid crystallayer are confined by rubbed polymer film, but excellent alignment inthe S_(A) phase is found no guarantee of achieving satisfactoryalignment in the S_(C) * phase. Particularly in the case of thin cells(having a liquid crystal layer thickness of about 2 microns), such asare typically employed for high speed switching, the density ofalignment defects can be extremely high in the S_(C) * phase. Suchdefects adversely affect both the persistence of switching and thecontrast ratio.

The wide occurrence of these defects is believed to be attributable inlarge part to the fact that the mechanism used to align the moleculardirector in the N* phase is liable to continue to exert an influence onmolecular alignment when the liquid crystal layer is in the S_(C) *phase. In the S_(C) * phase, however, the rubbing direction is stillnormal to the planes of the smectic layers, and hence is not a directionappropriate for the director in a tilted smectic phase. Strain istherefore associated with this form of alignment. If both major surfacesof the cell are similarly aligned the strain concentration is seen to beincreased as the liquid crystal layer thickness is reduced. It ispossible for a situation to arise when it is energetically morefavourable for the smectic layers to reorient than for the director toaccommodate all the strain within the layers. It is believed that thismay be a major factor contributing to the break-up of the layers in thincells. Another contributing factor may be small misalignments of rubbingdirection between the two major surfaces confining the liquid crystallayer.

Investigations have been made to see if the problem of the break-up ofthe layers is alleviated by constructing cells where only one of theconfining surfaces is provided by a rubbed polymer layer instead of bothsurfaces. We have found that this can be the case with cells in whichone confining surface is provided by a rubbed polymer coveredtransparent electroded glass substrate, while the other confiningsurface is provided by a similar substrate without the rubbed polymerlayer. However, it has been found that the quality of the result iscritically dependent upon the absence of blemishes in the surface of thesecond substrate. Preparation of satisfactory cells was found to bedifficult, time consuming, and unpredictable. Much better results havebeen obtained by adopting the teachings of the present invention.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided aferroeletric liquid crystal cell having a liquid crystal layer containedwithin an envelope, in which cell, for the promotion of parallelalignment of the molecules of the liquid crystal layer at least one ofits two major surfaces, said at least one major surface is in contactwith an associated polymer layer having a molecular structure withflexible side chains.

It is surmised that the flexible side chains of the polymer assist inthe promotion of uniform planar alignment without providing anypreferential azimuthal direction for that alignment.

If, instead of having rubbed polymer surfaces in contact with both majorsurfaces of the liquid crystal layer, only one major surface is incontact with a rubbed polymer surface, while the other major surfacewith a polymer with flexible side chains, the bookshelf alignment canstill be produced by the method described above involving the slowcooling of the liquid crystal layer through N* and S_(A) phases. But nowthe strain on entering an inclined smectic phase is less than beforebecause the flexible side chain polymer layer can relatively freelyaccommodate the reorientation of the molecular director of nearby liquidcrystal molecules upon their entering this phase. Some strain stillremains as a result of the constraints imposed by the rubbed polymercontacting the other major surface of the liquid crystal layer. Thissource of strain can be removed by replacing not one but both of therubbed polymer layers with the flexible side chain layers. Under thesecircumstances bookshelf alignment can not be induced in the same way asbefore. An alternative way of achieving bookshelf alignment would be bythe known method involving the application of a small amount ofmechanical shear to the liquid crystal layer by relative linear movementof its confining surfaces.

BRIEF DESCRIPTION OF THE DRAWING

There follows a description of a liquid crystal cell embodying theinvention in a preferred form. The description refers to theaccompanying drawing depicting a perspective schematic view of the cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A hermetically sealed envelope for a liquid crystal layer is formed bysecuring together two glass sheets 11 and 12 with a perimeter seal 13.The inward facing surfaces of the two sheets carry transparent electrodelayers (not shown) of indium tin oxide, and these are in turn coveredwith polymer layers 14 and 15. The thickness of the liquid crystal layercontained within the resulting envelope is dtermined by the thickness ofthe perimeter seal 13, and the thickness of this is determined by alight scattering of plastics spheres of uniform diameter over the areaenclosed by the perimeter seal. (Some of these spheres may also becomeincorporated into the material of the perimeter seal itself). Typicallythe thickness of such a cell may be about 2 um. Conveniently the cell isfilled by applying a vacuum to the interior of the envelope via anaperture (not shown) through one of the glass sheets in one corner ofthe are enclosed by the perimeter seal so as to cause the liquid crystalmedium to enter the cell via another aperture (not shown) located in thediagonally opposite corner. The filling operation is carried out withthe filling material heated ito its isotropic phase so as to reduce itsviscosity to a suitably low value. Subsequent to the filling operationthe two apertures are sealed.

The polymer layers 14 and 15 are provided for liquid crystal molecularalignment purposes. Polymer layer 14 is a rubbed polymer layer ofconventional type for molecular alignment, and may for instance be anylon layer. The other polymer layer, layer 15, is a liquid crystalpolymer constituted by a polysiloxane backbone with cyano-biphenylbenzoate side chain groups and having the general formula: ##STR1##

This liquid crystal polymer was applied from solution in the N-methylpyrolidone to the transparent electroded glass substrate 12 by spinning.This substrate 12, and the other transparent electroded glass substrate11 provided with a rubbed nylon coating, were sealed together with apolymer edge seal 13 to form a cell for filling with a proprietoryferroelectric liquid crystal mixture designated M622 developed by BDHwith the following characteristics:

    S.sub.C *-74° C.-S.sub.A -105° C.-N*-136° C.-I

In the foregoing it has been explained that the substitution of onerubbed polymer aligment layer of a ferroelectric liquid crystal layer byone of those unrubbed polymer layers with flexible side chains affords areduction in the stain accommodated by the liquid crystal layer. Afurther reduction of such strain is clearly also achievable byreplacement not only of the one rubbed surface, but of both rubbedsurfaces, provided that the bookshelf alignment can still be induced andmaintained by some other means.

We claim:
 1. A ferroelectric liquid crystal cell having a liquid crystallayer contained within an envelope, in which cell, for the promotion ofparallel alignment of the molecules of the liquid crystal layer at atleast one of its two major surfaces, at least one of said major surfacesis in contact with an associated polymer layer having a molecularstructure with flexible side chains of sufficient length and flexibilityto provide no preferential azimuthal direction of alignment for themolecules of the liquid crystal layer in contact therewith.
 2. Aferroelectric liquid crystal cell as claimed in claim 1 wherein thematerial of said polymer layer is a liquid crystal polymer.
 3. Aferroelectric liquid crystal cell as claimed in claim 1 wherein onemajor surface of the liquid crystal layer is in contact with a polymerlayer having said molecular structure with flexible side chains, whilethe other major surface is in contact with a polymer layer that has beenrubbed in order to promote planar alignment of the molecules of theliquid crystal layer in the rubbing direction at least when thosemolecules are present in an untilted liquid crystal phase.